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INVERTED CHEMISTRY Musings on Extra-Terrestrial Life One of a Kind? Last time we checked, the count for confirmed exoplanets was in the many thousands. Only a very few of those appear to be remotely Earth-like. While this may be a disappointment for those hoping to find "life as we know it", the Consular Science section reminds us that there are other option. The Creche The most compelling model for abiogenisis on the prinordial Earth is warm, alkaline seeps in the deep ocean beds, laden with organic molecules. welling up into seawater that was highly acidic due to very high CO2 content of the atmosphere in those days. This interface would have produced aggressive precipitation of insoluable metal salts crytals, which would have formed membranes that would have been the perfect templates for organic molecular evolution. Meanwhile, the pH gradient across these membranes would have provided the energy to drive that evolution. We see the echoes of this in our current cellular biology: Cell interiors maintained generally alkaline, with limited exposure to the 'acidic' (nowadays, from oxygen) environment. Unfortunately, this model would not appear to be likely on any of the exoplanets we have observed to date. Big Brothers Most of the planets in other stars "Goldilocks zones" are too small to be likely to hold an atmosphere. But there is another kind, less common, but still more common than Earth: The "Hycean" worlds. These are larger than Earth, 2 to 3 times. And postulated to be likely to have held onto significantly more water than our on world. That means a water-world, with a surface covered in ocean, perhaps miles deep. And that's actually a problem for the 'alkaline seep' model. You see, that model depends on the seeps venting into an ocean rendered acidic by a 50+ bar CO2 atmosphere. But a Hycean world, having a higher gravity, will have held onto much more volatiles, and end up with a more reducing atmosphere; CO2 would tend to be reduced to methane. That means the alkaline seep would be entering a simliarly alkaline ocean. No precipitation to provide convenient surfaces; no pH gradient to drive chemistry. There is Another However, hope is not lost. It occurs to us that the 'alkaline seeps' have more famous cousins, the "black smokers": Much hotter undersea vents, that emit hotter, more acidic water. A Hycean world might easily have these as well. And these would vent into an alkaline sea. Same pH gradient; same percipitations. Just inverted. Life evolving from such a source would maintain acidic cell interiors, which would be powered by limited exposure to an alkaline (relatively hydrogen-rich) atmosphere.